1. Field of the Invention
The present invention relates generally to optical fibers. Specifically, the invention relates to a novel method for insertion of core material into a cladding material to form an optical fiber preform having a high quality core-clad interface for use in fabricating a low loss optical fiber.
2. Description of the Related Art
Single mode optical fibers that transmit both visible and infrared energy are desirable for use in long distance communications systems. Minimization of transmission loss is particularly important in preparing optical fibers for use in long distance applications. Conventional methods for fabricating optical fibers involve casting glass melts into glass preforms which are then drawn into optical fibers. Optical fibers drawn from glass preforms having crystallites and bubbles, particularly at the core-clad interface, exhibit undesirable scattering loss and reduced transmission capabilities. Preforms prepared from heavy metal fluoride glasses are particularly susceptible to crystallite formation upon exposure of the glasses to temperatures at or above crystallization temperatures.
Conventional methods for casting optical fiber preforms expose the core and cladding materials to temperatures exceeding crystallization temperatures during the addition of the core material to the cladding material. These methods involve cooling a cladding glass melt and a subsequent reheating of the cladding glass to temperatures greater than the crystallization temperature upon the addition of a core glass melt to the cladding glass. Suction casting is a conventional preform casting method wherein cladding glass is poured into a preheated mold having a reservoir at the lower end. As the cladding glass cools to the glass transition temperature, a large thermal contraction occurs which constricts the cladding glass volume in the reservoir thereby forming a cylindrical cladding tube in the mold. A core glass melt is poured onto the cladding glass and is drawn through the cladding tube by a suction effect. The temperature of the core glass melt at the time it is poured onto the cladding glass is typically at least about 300.degree. C. above the crystallization temperature which reheats the cladding glass at the core-clad interface to temperatures above the crystallization temperature. Thus, this method like other conventional casting methods results in the formation of undesirable crystallites at the core-clad interface. Suction casting and other conventional methods such as, build-in casting, rotational casting, and rod-in-tube casting for making optical fiber preforms are described in chapter 5 of Fluoride Glass Fiber Optics, Academic Press, Inc., edited by Ishivar D. Aggarwal and Grant Lu, pp. 223-227 (1991). See also, for example, U.S. Pat. Nos. 4,793,842, 5,106,400 and 5,160,521.
Preforms cast by conventional methods must be modified before being drawn into single mode fibers having desired core and cladding diameters. Examples of such modifications include stretching procedures at temperatures which further increase crystallite formation and multiple jacketing procedures. Casting methods which require pouring of a core melt into a cladding melt also result in the formation of undesirable air bubbles at the core of the preform.